Field of Invention
The invention relates generally to a method for reducing dynamic loads of cranes. More precisely, the invention relates to a method for reducing resonant vibrations and dynamic loads of cranes, whose horizontal and vertical motion of the pay load is controlled by a boom winch controlling the lulling motion of a pivoting boom and a hoist winch controlling the vertical distance between the boom tip and the pay load.
Background of the Technology
Offshore cranes are frequently used for sea lifts where the load is picked up from a floating supply vessel. Such lifts normally represents higher dynamic loads to the crane than a similar rig or platform lift where the load is lifted from the same structure as the crane base.
The potential high dynamic load related to sea lift is closely linked to the difference in vertical speed between the vessel and the crane. If the load is lifted off the vessel deck while the vessel is moving downwards, then the jerk can make the peak load of the crane exceeding the allowable maximum. The risk of dynamic overloading and damages therefore increase with increasing load and vessel motions.
A skilled crane operator can often reduce the peak loads by picking the load off the vessel at the optimal heave phase, that is, when the vertical speed difference between vessel and boom tip is low. However, because the vessel heave is a stochastic process leading to non-periodic and unpredictable heave motion and because the humans can make mistakes, there is still a risk that the crane can be overloaded.
The load chart, which defines maximum allowable crane loads at different boom radii and rig heave conditions, is chosen to lower this risk to acceptable levels. The limitations in the operational weather window means high costs as a result of more waiting on weather.
The purpose of the invention is to overcome or reduce at least one of the disadvantages of the prior art.